1. Field of the Invention
The present invention relates, in general, to the design and manufacture of camera modules, and, more particularly, to the design and manufacture of fixed-focus camera modules having a lens barrel-to-lens housing interface that maintains focus alignment or adjustments between one or more lenses in the barrel and one or more image sensors in the housing.
2. Relevant Background
Digital camera technology is being used in an increasing variety of mass-produced applications. A growing use of digital camera technology is incorporating or providing fixed-focus camera modules in consumer products such as wireless telephones, cell phones, personal digital assistants (PDAs), and other handheld electronic devices. While many consumers demand high-end functionality and quality, many consumers want the functions such as those provided by a digital camera but at affordable prices. Within the next few years, it is expected that more than 65 percent of cell phones will include cameras. Additionally, there are many companies that produce consumer products such as cell phones and PDAs, and this competition requires that components including camera modules be produced with high quality but at acceptable costs with lower per unit material and assembly costs. This is especially true for products in which the camera is a secondary component such as when the product is primarily a communication device.
Fixed-focus camera modules used in many consumer products generally include a lens for focusing incoming light onto an image sensor that detects an image and converts it into an electrical signal representation. An image processor manipulates the image signal into an image that is stored or displayed on a display screen. Camera modules also include a chassis and enclosure for mounting the various electronic and optical components and for protecting the components from particulate and spurious light contamination. Modern digital camera modules are expected to provide high-quality imaging at low cost. Image quality depends on the camera's optics, and in fixed-focus camera modules, the focus is often set as part of the assembly process. Conventional assembly processes for digital camera modules tend to be labor-intensive. In particular, the focusing is often a manual operation in which an operator sets and secures the focus of each individual unit. Typically, setting the focus requires high-precision positioning of the camera's lens relative to the image sensor.
In FIG. 1, a conventional, fixed-focus camera module assembly 100 is shown that may be used to provide digital imaging functionality in consumer product or application such as a wireless or cellular phone. The module 100 is a stacked-die version in which the image sensor 124 is on top of a signal-processor 128. The lens system such as for a megapixel module is provided with a lens barrel 110 with one to three or more lens elements 112, with the number of lenses varying with optical design requirements. With a threaded interface 114, the lens barrel 110 is mounted to a housing 120, and the housing is shown to support an infrared (IR) filter 122 that filters longer-wavelength radiation to limit the noise created in the image sensor 124. A flexible circuit 130 with passive components is attached via solder joints 134 to the bottom of the processor 128 substrate. The components of module 100 are representative of many camera modules used today but many variations such as configuration, location, and number of the lenses 112 may be altered to provide designers and manufacturers alternative designs to suit criteria such as cost, size, shape, available technology, and other design parameters.
Assembly of the camera module 100 includes barrel insertion in which the barrel 110 is inserted into the housing 120. At a focusing station in the assembly process, the lens barrel 110 is rotated on the threaded interface 114 such that the lens elements 112 are focused accurately upon the image sensor 124. The focus may be tested electronically to obtain high quality results. A technician typically will apply two or more drops of adhesive 116, 117 after the focusing process is complete to try to retain the position of the barrel within the housing 120. The camera module 100 is then moved to a separate station along the assembly line to cure the adhesive 116, 117 such as in an oven or the like.
As shown in FIG. 1, conventional camera modules have included high-precision structural features that facilitate setting the proper focus, such as a lens assembly or lens barrel that attaches to the camera housing with screw threads. The threaded attachment provides a mechanism for positioning the focal point on the image sensor, and maintaining planarity between the lens and image sensor. During assembly of a conventional digital camera module, an operator threads the lens assembly onto the camera housing until a proper focus is achieved. The operator then temporarily fixes the threaded lens assembly positioning with drops of adhesive.
For example, the adhesive may be an ultraviolet (UV) glue or adhesive that is used to hold the position of the lens barrel relative to the lens housing once the module is focused in a focusing station by rotating the lens barrel. Proper focus requires that this focus position of the barrel be maintained with even slight movement of the barrel potentially ruining focus or at least reducing quality of resulting images. Presently, the camera module with the unset or partially-cured UV adhesive is moved to a heat curing oven for final cure. Unfortunately, the lens barrel often moves during the transfer of the camera module to the curing station or oven. This can result in the focus shifting, which can cause focus failure and poor yield. In other words, existing manufacturing processes allow focus shift between the initial application of the adhesive and stage in which the adhesive is fully cured. If the UV light cannot penetrate the total glue bond between the barrel and the housing, the adhesive will not totally cure, which could lead to focus shift later in the process. For example, heat from the UV cure oven may cause the focus to shift due to expansion and contraction.
As mass-produced products, camera modules are preferably constructed using low-cost materials such as plastics for as many components as possible including the lens barrel and the housing. However, the use of threads for establishing a proper focus for each manufactured digital camera module limits the ability to use lower cost construction materials as only certain types of materials can support machining and/or high precision molding. These materials are typically more expensive than the materials that are unable to support such processes. Also, it is desirable to minimize per unit costs associated with the assembly process, and the use of a post-focusing stage for curing adhesive increases the cycle time for each camera module as well as requiring additional equipment (i.e., curing ovens and associated equipment) and floor space. Particulate residue from the machining process can remain on the threaded parts and can potentially settle on the image sensor causing failure or lowering quality of digital images produced with the camera module. The use of adhesive also increases the risk of contaminating the interior of the module. Particulate residue is also generated by friction between the threads when they are rotated to set focus.
Hence, there remains a need for improved methods of maintaining focus of a camera module. Such methods may include providing an alternative configuration of the camera module to provide an interface between a lens barrel and a lens housing that provides an alternative to the standard threaded interface and that lends itself to improved barrel-to-housing bonding.